Rechargeable batteries are used in many applications today. One such application is the use of batteries in hybrid or fully electric vehicles. Within these vehicles, a plurality of individual battery cells are arranged in series in order to build a battery stack having a desired output voltage. A large number of cells may be arranged in series such that, for example, the total potential difference developed across the battery stack is in the order of several hundred volts. Each cell typically only has a potential difference of a few volts (say 2 to 4 volts) developed across it. Although the cells are similar, they are not identical, so repeated charging and discharging cycles may develop unequal voltages across individual cells within the stack. Ideally, the voltage across each individual cell, or at least small group of cells, would be monitored such that the cells could be temporarily removed from a charging process if their terminal voltage gets too high or, alternatively, if the cell temperature becomes unduly elevated. It is also possible to preferentially discharge cells to reduce their voltage. Whilst it is feasible to build a single battery monitoring apparatus that can operate across the entire voltage range, for example 0 to 400 volts, developed across a stack, such devices are expensive.
FIG. 1 illustrates a battery monitoring system of the related '615 application that includes a plurality of battery monitors 30-40 provided in association with respective battery cell groups 10-20. Each battery monitor tests the voltages of its respective battery cell group and reports the voltage values to a system controller 70. The battery monitors 30-40 are provided in a so-called ‘daisy chain’ integrated circuits in which data to be read from given battery monitor (say, monitor 40) is be passed serially from each battery monitor to the next (38→36→34, etc.) until it reaches a battery monitor 30 at the end of the chain. Battery monitor 30 may pass the communicated data further to a system controller 70.
Data can be read from battery monitors 30-40 from any position of the daisy chain and communicated to the system controller 70. Moreover, data can be transmitted from the system controller 70 to a battery monitor 30-40 at any position of the daisy chain. Thus, the system controller 70 can read or write data to any position of the daisy chain and battery monitors at intermediate positions of the daisy chain will relay the communicated data from the source of the data to its destination.
Each battery monitor part may have a different reference value to test the cell voltage level, which could lead to measurement errors. Temperature, for example, can cause reference voltages in the battery monitor chips to change. Since each battery monitor measures a distinct cell group, measurement variations along cell groups may occur. For example, cell 10 and cell 18 may actually be at the same voltage, but reference voltage variations may cause the battery monitor parts to report different voltage readings of the cells. The voltage reading variations may lead to a significant statistical offset in the cell group measurements. One solution is to provide an external reference voltage for each battery monitor chip. However, this solution is extremely expensive for practical purposes because of the high cost of external reference voltages. Therefore, there is a need in the art to reduce battery monitor chip to chip variations while keeping the solution cost efficient.